The present invention relates to tactile sensor apparatus and methods for operating them. The basic principle of a tactile sensor, also called a touch sensor, is to measure a contact between an object and a touch area, which is the sensory surface of the tactile sensor. In contrast to an industrial pressure or force sensor, a sensor signal which is directly proportional to the applied force is not required. Rather, the emphasis is generally on cost-effective coverage of a large or oddly-shaped area.
Primarily four approaches for implementing a large-area tactile sensor have been disclosed to date. In one approach, the touch area corresponds to a fixed plate suspended movably at a number of points, with the result that switching contacts are actuated in the event of force being applied. Neither an intensity nor a location of the pressure can be measured, and if the touch area is large it is not possible to achieve a high degree of sensitivity for the sensor. In another approach, the touch area is covered with an elastic and air-permeable material situated in an airtight sleeve, and air pressure sensors are fitted within the airtight sleeve. This approach does not allow spatial resolution of the force loading. In yet another approach, a pressure-sensitive membrane is stretched across the touch area according to a capacitive or piezoelectric principle. With this approach, it is generally possible only to detect a change in the force loading. According to still another approach, in a membrane-type pressure sensor, a conductive plastic material is applied to an interdigital structure, with an increasing pressure on the touch area resulting in a decreasing resistance between the two electrode combs. It is also possible to use further layers, for example for coverage and insulation. A membrane-type pressure sensor is comparatively costly to produce.
A still further example of a prior approach is found in a catalog from MicroTouch Systems describing a writing panel which operates according to the resistive sensor principle. To that end, a thin lower polyester layer is fixed on a rigid support, and a thin upper polyester layer is clamped above it. The upper polyester layer is under a tensile stress and is separated from the lower polyester layer by a gap. The mutually opposite surfaces of the two polyester layers are coated with a conductive material. In the event of pressure loading on the upper polyester layer, the latter is pressed onto the lower polyester layer and an electrical contact is closed, the position of which can be determined.
An object of the present invention is to provide a versatile and cost-effective tactile sensor having a simple structure. This object is achieved by means of a tactile sensor and by means of methods for operating it, in accordance with specific embodiments of the present invention.
According to a specific embodiment, the tactile sensor has at least two conductive workpieces which lie one on top of the other at a common bearing area. The workpieces are thus in mechanical contact with one another at the bearing area. Of the at least two conductive workpieces, at least one workpiece is composed of conductive elastomer material (called an elastomer piece). The elastomer piece is both elastic and conductive. In the event of loading of a force F on the elastomer piece, with the result that the latter is pressed against the other workpiece, there is a change in the contact resistance or the surface transition conductance at that part (called the contact area) of the bearing area that is exposed to pressure. The contact resistance is generally dependent on the contact area and on the applied pressure, while the electric conductance in the internal volume of the elastomer piece is only slightly dependent on the mechanical load. Typically, as the contact resistance decreases, the larger the contact area is and the greater the pressure is.
In contrast to the membrane-type pressure sensor, the tactile sensor of the present invention requires just one, additionally extended, bearing area between two workpieces. Unlike the sensor from MicroTouch, the workpiece of the present invention does not have to be mechanically clamped. According to the present invention, the mechanical bearing of the workpieces means that there is also no need to perform complicated setting of a distance. Moreover, unlike the membrane-type pressure sensor and the sensor from MicroTouch, the bearing area of the present tactile sensor is not restricted to a planar form. Furthermore, the need to structure the bearing area is obviated in the case of the tactile sensor. The workpieces and the bearing area may also have cutouts. It is sufficient if one of the workpieces is produced from elastomer material, for some embodiments. The other material may be made for example of metal, e.g., a metal sheet or a foil. However, such an arrangement affords a limited spatial resolution and flexibility.
With the present invention, an application of force can be measured for example by means of measuring a current flow through the contact area or a resistance value that takes account of the contact resistance. To that end, the workpieces may, for example, be equipped with electrodes. A voltage loading and/or measurement of a contact resistance can be done for example by means of electrodes connected to the workpieces, e.g., electrodes incorporated in the elastomer, or by other means for electrical contact with the workpieces. The means for electrical contact is referred to below as electrode, in a manner that does not constitute a restriction. A sum of resistances is measured, inter alia a transition resistance from the electrode to the workpiece a series resistance along or through the workpiece, and a contact resistance between one workpiece and the other. The composition of the resistance values for a given contact situation is known for every mechanical arrangement, with the result that the contact resistance sought can be calculated. In addition to information about an intensity of the external force given suitable circuitry, it is also possible to determine the position of the contact area and its size.